**Acknowledgement**

Financial support provided by the Academy of Finland (project no.139571) is gratefully acknowledged. Molecular graphics was created with the UCSF Chimera package. Chimera is developed by the Resource for Biocomputing, Visualization, and Informatics at the University of California, San Francisco, with support from the National Institutes of Health (National Center for Research Resources grant 2P41RR001081, National Institute of General Medical Sciences grant 9P41GM103311).
